1. Field of the Invention
The present invention relates to semiconductor devices having fuses and, more particularly, to memory devices having spare (i.e. redundant) memory cells, and fuse-link programmable read-only memory (PROM) devices.
2. Description of the Prior Art
A semiconductor memory is organized as a matrix of horizontal or row lines and vertical or column lines. At each intersection of the matrix a memory cell is provided which is able to store one bit of information. If at least one defect, e.g. a defective memory cell, occurs in a memory device, the device should be abandoned. However, if the number of the defects is small, the bad memory device can be changed into a good memory device by using a redundancy technique. It is possible to remarkably increase the production yield by providing redundant (i.e. spare) row and column lines in the memory device. The spare row lines and spare column lines which form spare memory cells are built into the memory device while it is being produced. When, for example, a row line of a defective memory cell (i.e. a defective bit) is found during testing, the defective row line is eliminated and a spare row line is substituted in its place. Namely, the defective row line is cut off from the regular row decoder thereof and, simultaneously, the spare row decoder of the spare row line is formed to respond to the same address as that of the defective row line to be replaced. Thus, the spare row decoder behaves exactly like the regular row decoder. The above-mentioned cutting off of the defective row line and formation of the spare row decoder can be carried out by blowing fuses provided between the regular row decoder and the defective row line and within the spare row decoder. Similarly, a defective column line can be eliminated and a spare column line can be substituted in its place. Generally, fuses are formed in every row line, every column line, every spare row decoder and every spare column decoder of a semiconductor memory device.
There is a well-known fuse-link-programmable read-only memory (PROM) device provided with fuses in every bit (i.e. memory cell). In order to write information, a fuse in a bit is blown by passing excess current (i.e. fusing current) between a row line and a column line of the bit.
A fuse element comprising both the above-mentioned fuses of the semiconductor memory device which has redundant memory cells and the fuses of the fuse-link PROM has a structure as illustrated in FIG. 1. The fuse element 1 of FIG. 1 is produced in the following manner. The starting material is a semiconductor substrate (e.g. silicon wafer) 2. An isolating layer (e.g. silicon dioxide layer) 3 is formed on the semiconductor substrate 2 by chemical vapor deposition or thermal oxidation of silicon. Fuse material, e.g. polysilicon, nichrome (Ni-Cr), titanium-tungsten (Ti-W) or platinum silicide (PtSi), is deposited on the isolating layer 3 by vapor deposition or sputtering and, then, is patterned by photoetching to form a fuse 4 having a so-called bow-tie shape which consists of two relatively large rectangular portions and a belt shaped portion connecting the two rectangular portions. An insulating layer 5 of silicon dioxide (SiO.sub.2), silicon nitride (SiN.sub.4) or phospho-silicate glass (PSG) is formed on the fuse 4 and the isolating layer 3 by chemical vapor deposition and, then, is selectively etched to open two windows for electrical contact on the fuse 4 by photoetching. Then, a conductor material (e.g. aluminum) is formed by vapor deposition and is selectively photo etched to form predetermined conductor lines 6. A protective layer 7 of silicon dioxide, silicon nitride or phospho-silicate glass is formed on the conductor lines 6 and the insulating layer 5.
Fusing of the fuse 4 is performed by passing a fusing current through the fuse 4 via the conductor lines 6. The fusing current is limited to a value of current which is allowed to pass through a metal oxide semiconductor (MOS) transistor or a bipolar transistor in a memory device. However, it is possible that a fuse can not be blown by passing this level of fusing current and/or that a small bridge is formed between separated portions of the blown fuse, and therefore, fuse-reliability is low.